Endomysium: Surrounds muscle fibers (cells) within a fascicle.
Skeletal Muscle Fibers
Skeletal muscle fiber = muscle cell
Multinucleated.
Sarcolemma: cell membrane
Sarcoplasm: cytoplasm
Many myofibrils.
Myofibrils consist of myofilaments proteins:
Thin actin filaments.
Thick myosin filaments
Sarcomeres: units of action
Sarcoplasmic reticulum: Stores Calcium ions
Transverse (‘T’) tubule: channels to distribute calcium ions. (invagination of the sarcolemma that helps internalize the muscle impulse
Myofibrils and Sarcomeres
Myofibrils consist of sarcomeres connected end-to-end.
The striation pattern is due to the arrangement of myofilaments in myofibrils.
Sarcomeres contain these structures:
I band (thin filaments).
A band (thick & thin filaments).
H zone (thick filaments).
Z line (or Z disc).
M line.
Striation Pattern
Striation pattern has 2 main parts:
I Band:
Light band
Composed of thin actin filaments only
A Band:
Dark band
Composed of thick myosin filaments with portions overlapped with thin actin filaments
H Zone:
Center of A band
Composed of thick myosin filaments
Z Line:
Anchors filaments in place
Sarcomere boundary
Center of I band
M Line:
Anchors thick filaments
Center of A band
Myofilaments
Thick filaments:
Composed of two strands of myosin protein with projecting heads that form cross-bridges
Thin filaments:
Composed of actin protein
Associated with troponin and tropomyosin, which prevent cross-bridge formation when the muscle is not contracting
Neuromuscular Junction (NMJ)
Neuromuscular Junction (NMJ):
A type of synapse.
Site where an axon of motor neuron and skeletal muscle fiber interact.
Skeletal muscle fibers contract only when stimulated by a motor neuron.
Parts of a NMJ:
Motor neuron.
Motor end plate.
Synaptic cleft.
Synaptic vesicles.
Neurotransmitters.
Neurotransmitters and Muscle Contraction
Acetylcholine (ACh) is the neurotransmitter.
Nerve impulse causes the release of ACh from synaptic vesicles.
ACh binds to ACh receptors on the motor end plate.
ACh causes changes in membrane permeability.
Stimulus for Contraction: Which generates a muscle impulse (action potential)
Impulse causes release of Ca^{+2} from SR, which leads to muscle contraction
Muscle Contraction Process
At NMJ: Signal from the brain to motor neuron to nerve axon terminal $\rightarrow$ Release of Acetylcholine from synaptic vesicle to synaptic cleft $\rightarrow$ Acetylcholine binds to Acetylcholine receptors on motor endplate.
All around the sarcolemma: The sarcolemma is stimulated. An impulse travels over the surface of the muscle fiber and deep into the fiber through the transverse tubules. The impulse reaches the sarcoplasmic reticulum, and calcium channels open.
Calcium ions diffuse from the sarcoplasmic reticulum into the cytosol and bind to troponin molecules.
Inside the Sarcoplasm
Calcium ions bind to troponin, causing a change in its shape.
Each tropomyosin molecule is held in place by a troponin molecule. The change in shape of troponin changes the position of tropomyosin.
Binding sites on actin are now exposed.
Myosin heads bind to actin, forming cross-bridges.
Excitation-Contraction Coupling
Excitation-Contraction Coupling:
Connection between muscle fiber stimulation and muscle contraction
Release of Ca^{+2} from sarcoplasmic reticulum exposes binding sites on thin filament:
Ca^{+2} binds to troponin.
Tropomyosin pulled aside.
Binding sites on thin filament exposed.
Exposed binding sites on actin allow the muscle contraction cycle to occur.
Myosin heads bind to actin, forming cross-bridges, connecting myosin to actin.
ADP and P release from myosin and cross-bridge pulls thin filament (power stroke).
New ATP binds to myosin, breaking the connection to actin.
ATP splits, which provides power to "cock" the myosin heads and store energy for the next power stroke.
Active transport of Ca^{+2} into sarcoplasmic reticulum, which requires ATP, makes myosin-binding sites unavailable.
If the stimulus for contraction is maintained, Ca^{+2} continues to bind to troponin.
Sliding Filament Model of Muscle Contraction
Sliding Filament Model of Muscle Contraction:
When sarcomeres shorten, thick and thin filaments slide past one another.
H zones and I bands narrow.
Z lines move closer together.
Thin and thick filaments do not change length.
Overlap between filaments increases.
Relaxation
When neural stimulation of the muscle fiber stops:
Acetylcholinesterase (enzyme) rapidly decomposes ACh remaining in the synapse.
The muscle impulse stops when ACh is decomposed.
Stimulus to sarcolemma and muscle fiber membrane ceases.
The calcium pump moves Ca^{+2} back into the sarcoplasmic reticulum (SR).
The troponin-tropomyosin complex again covers the binding sites on actin.
Myosin and actin binding are now prevented.
Muscle fiber relaxes.
Both contraction & relaxation use ATP (need energy)
Major Events of Muscle Fiber Contraction
An impulse travels down a motor neuron axon.
The motor neuron releases the neurotransmitter acetylcholine (ACh).
ACh binds to ACh receptors in the muscle fiber membrane.
The sarcolemma is stimulated. An impulse travels over the surface of the muscle fiber and deep into the fiber through the transverse tubules.
The impulse reaches the sarcoplasmic reticulum, and calcium channels open.
Calcium ions diffuse from the sarcoplasmic reticulum into the cytosol and bind to troponin molecules.
Tropomyosin molecules move and expose specific sites on actin where myosin heads can bind.
Cross-bridges form, linking thin and thick filaments.
Thin filaments are pulled toward the center of the sarcomere by pulling of the cross-bridges.
The muscle fiber exerts a pulling force on its attachments as a contraction occurs.
Major Events of Muscle Fiber Relaxation
Acetylcholinesterase decomposes acetylcholine, and the muscle fiber membrane is no longer stimulated.
Calcium ions are actively transported back into the sarcoplasmic reticulum.
ATP breaks cross-bridge linkages between actin and myosin filaments without breakdown of the ATP itself.
Breakdown of ATP “cocks” the myosin heads.
Troponin and tropomyosin molecules block the interaction between myosin and actin filaments.
The muscle fiber remains relaxed, yet ready, until stimulated again.
Energy Sources for Contraction
ATP reserves: Small amount, and so must be regenerated.
Creatine phosphate: Stores excess ATPs. CP initially regenerates ATP from ADP and phosphate.
Cellular respiration.
Cellular Respiration
Glycolysis, Anaerobic: yields 2 ATP
Citric Acid Cycle, Aerobic
Electron Transport Chain, Aerobic
All steps yield 32 to 36 ATP
Hemoglobin in red blood cells carries oxygen to muscle tissue.
The pigment myoglobin stores oxygen in muscle tissue for aerobic respiration; this increases oxygen availability.
Oxygen Debt
During rest or moderate exercise, respiratory & cardiovascular systems supply enough O_2 to support aerobic respiration
Anaerobic (Lactic Acid) Threshold: Shift in metabolism from aerobic to anaerobic, during strenuous muscle activity, when the above systems cannot supply the necessary O_2. Lactic acid is produced.
Oxygen Debt: also called excess post-exercise oxygen consumption
Amount of oxygen needed by liver cells to convert the accumulated lactic acid to glucose, and to restore muscle ATP and creatine phosphate concentrations.
Physical training helps increase a muscle’s capacity to improve energy production